The production of glass sheet often requires rolls for pulling, supporting, and conveying the sheet at elevated temperatures. The glass will often have a temperature in excess of 500° C. and frequently in excess of 650° C. Rolls must be capable of withstanding such operating temperatures for prolonged periods. Failure of the roll in a continuous production process can be very costly in time, man-power, material and lost revenue. The rolls should therefore resist thermally degradation, mechanically erosion, or dimensional changes, and should not negatively affect the glass.
Rolls may support or convey a glass sheet through an annealing or heat treating furnace. Rolls may also flatten, lengthen or otherwise alter the dimensions of the glass. A roll may even generate a pulling force on the glass to control the glass thickness. In any application, the roll should not contaminate the useable surface of the glass or produce an excessive number of onclusions. Onclusions can occur from “dusting” of the roll, that is, when small particles erode from the roll and stick to the glass. Onclusions are more likely to form on hot glass, such as around pulling rolls right out of the furnace.
Rolls may comprise an outer refractory body bonded to an inner metal shaft. The refractory body resists thermal insults and protects the metal shaft from heat. The metal shaft provides mechanical strength to the refractory body. In one such embodiment, a tubular outer refractory body is cemented to a metal shaft. This unitary structure is strong and simple to produce. Although the metal shaft is insulated from the high temperature glass, damage to any part of the roll requires replacement of the entire roll. Repair of only part of the roll is difficult or impossible. Other problems include cracking caused by mismatches in thermal expansion between the metal shaft, the cement, and the refractory body. The metal shaft expands more than the outer refractory body and exerts a tensile stress on the refractory body. Tensile stresses are particularly damaging because the refractory body is commonly a ceramic, and ceramics are typically weak in tension. Water cooling may be used to reduce the temperature of the metal shaft and therefore its expansion. Unfortunately, the fittings necessary for water cooling add additional expense and complexity to the roll.
A popular roll for use in glass manufacture had included a plurality of asbestos fiber discs stacked over a metal shaft. The asbestos discs were laterally compressed to form a rigid outer surface. The erosion-resistance of the surface could even be improved by impregnation with chemicals such as potassium sulfate. Unlike unitary structures, damage to one or several asbestos discs could be repaired by replacing only the damaged discs. Asbestos fiber is resilient and a good insulator, so it both thermally shielded the metal shaft and accommodated any thermal expansion of the metal shaft that might have occurred. Asbestos also had little affinity for glass, so eroded particles did not stick to glass or form onclusions. Of course, the health risks of asbestos prevent its use. Other ceramic fibers have been used in place of asbestos but such fibers are not as refractory, thermally insulating or erosion-resistant, and may share similar health risks. Further, eroded ceramic particles may adhere to the glass, thereby forming onclusions. Silica particles are particularly susceptible to onclusion formation.
Prior art includes rolls that reduce the erodable surface of the roll. Such rolls may comprise a metal shaft having a plurality of refractory collars. This configuration may be useful in those applications, such as pulling rolls, where only a portion of the glass contacts the roll. A large fraction of the metal shaft is left uncovered by a refractory body. Eliminating the refractory body removes a possible source of dusting and onclusions, but the exposed metal shaft is more susceptible to corrosion and dimensional instability when exposed to elevated temperatures, which may exceed 700° C. Corrosion may cause the metal shaft to break or deposit corrosion products on the glass. Dimensional changes in the roll can cause fracture or distortion of the glass. A coating may be applied to the metal shaft to reduce corrosion but the metal shaft still may warp from the high temperatures. The use of corrosion-resistant and more heat-tolerant metals, such as stainless steel, reduces this risk. Of course, this also increases costs and the metal still is substantially less refractory than a ceramic.
Rolls do not necessarily require a metal shaft for mechanical support. Prior art includes roll comprising a solid fused silica cylinder. Fused silica inherently has a very low coefficient of thermal expansion and has been used where thermal gradients are severe. Fused silica rolls do not corrode and are more dimensionally stable than rolls including metal shafts. Negatively, fused silica rolls do not grip glass sufficiently to function as pulling rolls, lack the strength of metal-shafted rolls, and cannot be directly connected to machinery for driving the rotation of the roll. Metal end caps, which are fixedly secured to the roll, permit mechanical connection to the driving machinery, but are not without their problems. The metal-capped ends must engage the driving machinery and transmit torque to the roll. Problems include securing the end caps permanently to the ceramic roll and loss of torque between the end cap and the roll. Thermal expansion disparities between the ceramic roll and the metal end cap contribute to both problems.
A need exists for a high temperature roll that overcomes the limitations of the prior art. The roll should be substantially non-dusting and should be suitable for use so as a pulling roll. The roll should possess good mechanical strength and accommodate any thermal expansion disparities between the materials. The roll should also possess excellent dimensional stability.